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man:tcp

TCP(7) Linux Programmer's Manual TCP(7)

NAME

     tcp - TCP protocol

SYNOPSIS

     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netinet/tcp.h>
     tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION

     This  is  an  implementation  of  the  TCP protocol defined in RFC 793,
     RFC 1122 and RFC 2001 with the NewReno and SACK  extensions.   It  pro-
     vides  a  reliable, stream-oriented, full-duplex connection between two
     sockets on top of ip(7), for both v4 and v6 versions.   TCP  guarantees
     that the data arrives in order and retransmits lost packets.  It gener-
     ates and checks a per-packet checksum  to  catch  transmission  errors.
     TCP does not preserve record boundaries.
     A  newly  created  TCP socket has no remote or local address and is not
     fully specified.  To create an outgoing TCP connection  use  connect(2)
     to establish a connection to another TCP socket.  To receive new incom-
     ing connections, first bind(2) the socket to a local address  and  port
     and  then  call  listen(2)  to put the socket into the listening state.
     After that a new socket for each incoming connection  can  be  accepted
     using  accept(2).   A socket which has had accept(2) or connect(2) suc-
     cessfully called on it is fully specified and may transmit data.   Data
     cannot be transmitted on listening or not yet connected sockets.
     Linux supports RFC 1323 TCP high performance extensions.  These include
     Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling  and
     Timestamps.   Window scaling allows the use of large (> 64 kB) TCP win-
     dows in order to support links with high latency or bandwidth.  To make
     use of them, the send and receive buffer sizes must be increased.  They
     can  be  set  globally   with   the   /proc/sys/net/ipv4/tcp_wmem   and
     /proc/sys/net/ipv4/tcp_rmem  files,  or  on individual sockets by using
     the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.
     The  maximum  sizes  for  socket buffers declared via the SO_SNDBUF and
     SO_RCVBUF   mechanisms   are   limited   by   the   values    in    the
     /proc/sys/net/core/rmem_max   and   /proc/sys/net/core/wmem_max  files.
     Note that TCP actually allocates twice the size of the buffer requested
     in  the setsockopt(2) call, and so a succeeding getsockopt(2) call will
     not return the same size of buffer as requested  in  the  setsockopt(2)
     call.   TCP uses the extra space for administrative purposes and inter-
     nal kernel structures, and the /proc file  values  reflect  the  larger
     sizes  compared  to the actual TCP windows.  On individual connections,
     the socket buffer size must be set prior to the listen(2) or connect(2)
     calls in order to have it take effect.  See socket(7) for more informa-
     tion.
     TCP supports urgent data.  Urgent data is used to signal  the  receiver
     that  some  important  message  is  part of the data stream and that it
     should be processed as soon as possible.  To send urgent  data  specify
     the  MSG_OOB option to send(2).  When urgent data is received, the ker-
     nel sends a SIGURG signal to the process or process group that has been
     set  as  the socket "owner" using the SIOCSPGRP or FIOSETOWN ioctls (or
     the POSIX.1-specified fcntl(2) F_SETOWN operation).  When the SO_OOBIN-
     LINE  socket option is enabled, urgent data is put into the normal data
     stream (a program can test for its location using the SIOCATMARK  ioctl
     described  below),  otherwise  it can be received only when the MSG_OOB
     flag is set for recv(2) or recvmsg(2).
     When out-of-band data is present, select(2) indicates the file descrip-
     tor as having an exceptional condition and poll (2) indicates a POLLPRI
     event.
     Linux 2.4 introduced a number of changes for  improved  throughput  and
     scaling,  as  well  as  enhanced functionality.  Some of these features
     include support for zero-copy sendfile(2), Explicit Congestion  Notifi-
     cation,  new management of TIME_WAIT sockets, keep-alive socket options
     and support for Duplicate SACK extensions.
 Address formats
     TCP is built on top of IP (see ip(7)).  The address formats defined  by
     ip(7)  apply  to  TCP.  TCP supports point-to-point communication only;
     broadcasting and multicasting are not supported.
 /proc interfaces
     System-wide TCP parameter settings can be  accessed  by  files  in  the
     directory  /proc/sys/net/ipv4/.   In addition, most IP /proc interfaces
     also apply to TCP; see ip(7).  Variables described as Boolean  take  an
     integer  value,  with  a nonzero value ("true") meaning that the corre-
     sponding option is enabled, and a zero value ("false") meaning that the
     option is disabled.
     tcp_abc (Integer; default: 0; Linux 2.6.15 to Linux 3.8)
            Control  the  Appropriate Byte Count (ABC), defined in RFC 3465.
            ABC is a way of increasing the  congestion  window  (cwnd)  more
            slowly  in response to partial acknowledgments.  Possible values
            are:
            0  increase cwnd once per acknowledgment (no ABC)
            1  increase cwnd once per acknowledgment of full sized segment
            2  allow increase cwnd by two if acknowledgment is of  two  seg-
               ments to compensate for delayed acknowledgments.
     tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
            Enable  resetting  connections  if  the listening service is too
            slow and unable to keep up and accept them.  It  means  that  if
            overflow  occurred  due to a burst, the connection will recover.
            Enable this option only if you are really sure that the  listen-
            ing  daemon  cannot  be  tuned  to  accept  connections  faster.
            Enabling this option can harm the clients of your server.
     tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
            Count  buffering  overhead  as   bytes/2^tcp_adv_win_scale,   if
            tcp_adv_win_scale    is    greater    than    0;    or    bytes-
            bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less  than
            or equal to zero.
            The  socket  receive buffer space is shared between the applica-
            tion and kernel.  TCP maintains part of the buffer  as  the  TCP
            window, this is the size of the receive window advertised to the
            other end.  The rest of the space is used as  the  "application"
            buffer, used to isolate the network from scheduling and applica-
            tion  latencies.   The  tcp_adv_win_scale  default  value  of  2
            implies  that  the  space used for the application buffer is one
            fourth that of the total.
     tcp_allowed_congestion_control (String; default: see text; since  Linux
     2.4.20)
            Show/set the congestion control algorithm choices  available  to
            unprivileged  processes  (see the description of the TCP_CONGES-
            TION socket option).  The items in the  list  are  separated  by
            white  space and terminated by a newline character.  The list is
            a subset of those  listed  in  tcp_available_congestion_control.
            The  default value for this list is "reno" plus the default set-
            ting of tcp_congestion_control.
     tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
            If this option is enabled, the kernel tries  to  coalesce  small
            writes  (from consecutive write(2) and sendmsg(2) calls) as much
            as possible, in order to decrease the total number of sent pack-
            ets.   Coalescing  is  done if at least one prior packet for the
            flow is waiting  in  Qdisc  queues  or  device  transmit  queue.
            Applications  can still use the TCP_CORK socket option to obtain
            optimal behavior when they know how/when to uncork  their  sock-
            ets.
     tcp_available_congestion_control   (String;   read-only;   since  Linux
     2.4.20)
            Show a list of the congestion-control algorithms that are regis-
            tered.  The items in the list are separated by white  space  and
            terminated  by a newline character.  This list is a limiting set
            for the list in  tcp_allowed_congestion_control.   More  conges-
            tion-control  algorithms  may  be  available as modules, but not
            loaded.
     tcp_app_win (integer; default: 31; since Linux 2.4)
            This variable defines how many  bytes  of  the  TCP  window  are
            reserved for buffering overhead.
            A maximum of (window/2^tcp_app_win, mss) bytes in the window are
            reserved for the application buffer.  A value of 0 implies  that
            no amount is reserved.
     tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
            The  initial value of search_low to be used by the packetization
            layer Path MTU discovery  (MTU  probing).   If  MTU  probing  is
            enabled, this is the initial MSS used by the connection.
     tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
            Enable  BIC  TCP  congestion  control  algorithm.   BIC-TCP is a
            sender-side-only change that ensures a linear RTT fairness under
            large  windows  while offering both scalability and bounded TCP-
            friendliness.  The protocol combines two schemes called additive
            increase and binary search increase.  When the congestion window
            is large, additive increase with a large increment ensures  lin-
            ear  RTT fairness as well as good scalability.  Under small con-
            gestion windows, binary search increase provides  TCP  friendli-
            ness.
     tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
            Set  the  threshold  window (in packets) where BIC TCP starts to
            adjust the congestion window.   Below  this  threshold  BIC  TCP
            behaves the same as the default TCP Reno.
     tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6
     to 2.6.13)
            Force  BIC  TCP to more quickly respond to changes in congestion
            window.  Allows two flows sharing the same  connection  to  con-
            verge more rapidly.
     tcp_congestion_control (String; default: see text; since Linux 2.4.13)
            Set  the default congestion-control algorithm to be used for new
            connections.  The algorithm  "reno"  is  always  available,  but
            additional choices may be available depending on kernel configu-
            ration.  The default value for this file is set as part of  ker-
            nel configuration.
     tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
            Lower  limit, in bytes, of the size of socket reads that will be
            offloaded to a DMA copy engine, if one is present in the  system
            and the kernel was configured with the CONFIG_NET_DMA option.
     tcp_dsack (Boolean; default: enabled; since Linux 2.4)
            Enable RFC 2883 TCP Duplicate SACK support.
     tcp_ecn (Integer; default: see below; since Linux 2.4)
            Enable RFC 3168 Explicit Congestion Notification.
            This file can have one of the following values:
            0      Disable  ECN.  Neither initiate nor accept ECN.  This was
                   the default up to and including Linux 2.6.30.
            1      Enable ECN when requested  by  incoming  connections  and
                   also request ECN on outgoing connection attempts.
            2      Enable ECN when requested by incoming connections, but do
                   not request ECN on outgoing connections.  This  value  is
                   supported, and is the default, since Linux 2.6.31.
            When   enabled,  connectivity  to  some  destinations  could  be
            affected due to older, misbehaving middle boxes along the  path,
            causing  connections  to be dropped.  However, to facilitate and
            encourage deployment with option 1,  and  to  work  around  such
            buggy  equipment,  the  tcp_ecn_fallback  option has been intro-
            duced.
     tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
            Enable RFC 3168, Section 6.1.1.1. fallback.  When enabled,  out-
            going  ECN-setup  SYNs  that  time  out  within  the  normal SYN
            retransmission timeout will be resent with CWR and ECE  cleared.
     tcp_fack (Boolean; default: enabled; since Linux 2.2)
            Enable TCP Forward Acknowledgement support.
     tcp_fin_timeout (integer; default: 60; since Linux 2.2)
            This  specifies  how many seconds to wait for a final FIN packet
            before the socket is forcibly closed.  This is strictly a viola-
            tion  of  the TCP specification, but required to prevent denial-
            of-service attacks.  In Linux 2.2, the default value was 180.
     tcp_frto (integer; default: see below; since Linux 2.4.21/2.6)
            Enable F-RTO, an enhanced recovery algorithm for TCP retransmis-
            sion timeouts (RTOs).  It is particularly beneficial in wireless
            environments where packet loss is typically due to random  radio
            interference  rather  than  intermediate router congestion.  See
            RFC 4138 for more details.
            This file can have one of the following values:
            0  Disabled.  This was the default up  to  and  including  Linux
               2.6.23.
            1  The basic version F-RTO algorithm is enabled.
            2  Enable SACK-enhanced F-RTO if flow uses SACK.  The basic ver-
               sion can be used also when SACK is in use though in that case
               scenario(s)  exists  where  F-RTO  interacts  badly  with the
               packet counting of the SACK-enabled TCP flow.  This value  is
               the default since Linux 2.6.24.
            Before  Linux  2.6.22,  this parameter was a Boolean value, sup-
            porting just values 0 and 1 above.
     tcp_frto_response (integer; default: 0; since Linux 2.6.22)
            When F-RTO has detected that a TCP  retransmission  timeout  was
            spurious  (i.e., the timeout would have been avoided had TCP set
            a longer retransmission timeout), TCP has several  options  con-
            cerning what to do next.  Possible values are:
            0  Rate  halving  based;  a  smooth  and  conservative response,
               results in halved congestion  window  (cwnd)  and  slow-start
               threshold (ssthresh) after one RTT.
            1  Very  conservative  response;  not  recommended  because even
               though being valid, it interacts  poorly  with  the  rest  of
               Linux TCP; halves cwnd and ssthresh immediately.
            2  Aggressive  response; undoes congestion-control measures that
               are now known to be unnecessary (ignoring the possibility  of
               a  lost retransmission that would require TCP to be more cau-
               tious); cwnd and ssthresh are restored to the values prior to
               timeout.
     tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
            The number of seconds between TCP keep-alive probes.
     tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
            The  maximum number of TCP keep-alive probes to send before giv-
            ing up and killing the connection if  no  response  is  obtained
            from the other end.
     tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
            The  number  of seconds a connection needs to be idle before TCP
            begins sending out keep-alive probes.  Keep-alives are sent only
            when  the  SO_KEEPALIVE  socket  option is enabled.  The default
            value is 7200 seconds (2 hours).  An idle connection  is  termi-
            nated  after approximately an additional 11 minutes (9 probes an
            interval of 75 seconds apart) when keep-alive is enabled.
            Note that underlying connection tracking mechanisms and applica-
            tion timeouts may be much shorter.
     tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
            If  enabled,  the  TCP  stack  makes decisions that prefer lower
            latency as opposed to higher throughput.  It this option is dis-
            abled,  then  higher  throughput is preferred.  An example of an
            application where this default should  be  changed  would  be  a
            Beowulf compute cluster.
     tcp_max_orphans (integer; default: see below; since Linux 2.4)
            The  maximum  number  of orphaned (not attached to any user file
            handle) TCP sockets allowed in the system.  When this number  is
            exceeded,  the  orphaned  connection  is  reset and a warning is
            printed.  This limit exists only to  prevent  simple  denial-of-
            service  attacks.  Lowering this limit is not recommended.  Net-
            work conditions might require you  to  increase  the  number  of
            orphans  allowed, but note that each orphan can eat up to ~64 kB
            of unswappable memory.  The default initial value is  set  equal
            to  the  kernel  parameter  NR_FILE.   This  initial  default is
            adjusted depending on the memory in the system.
     tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
            The maximum number of  queued  connection  requests  which  have
            still  not  received  an  acknowledgement  from  the  connecting
            client.  If this number is exceeded, the kernel will begin drop-
            ping  requests.   The  default value of 256 is increased to 1024
            when the memory present in the system is adequate or greater (>=
            128 MB), and reduced to 128 for those systems with very low mem-
            ory (<= 32 MB).
            Prior to Linux 2.6.20, it was recommended that if this needed to
            be  increased  above  1024,  the  size  of the SYNACK hash table
            (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep
                TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
            and the kernel should be recompiled.  In Linux 2.6.20, the fixed
            sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.
     tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
            The maximum number of sockets in TIME_WAIT state allowed in  the
            system.  This limit exists only to prevent simple denial-of-ser-
            vice attacks.   The  default  value  of  NR_FILE*2  is  adjusted
            depending  on  the  memory  in  the  system.   If this number is
            exceeded, the socket is closed and a warning is printed.
     tcp_moderate_rcvbuf   (Boolean;   default:   enabled;    since    Linux
     2.4.17/2.6.7)
            If enabled, TCP performs receive buffer auto-tuning,  attempting
            to  automatically  size the buffer (no greater than tcp_rmem[2])
            to match the size required by the path for full throughput.
     tcp_mem (since Linux 2.4)
            This is a vector of 3 integers: [low,  pressure,  high].   These
            bounds,  measured  in units of the system page size, are used by
            TCP to track its memory usage.  The defaults are  calculated  at
            boot  time  from  the amount of available memory.  (TCP can only
            use low  memory  for  this,  which  is  limited  to  around  900
            megabytes  on 32-bit systems.  64-bit systems do not suffer this
            limitation.)
            low       TCP doesn't regulate its memory  allocation  when  the
                      number  of  pages  it  has allocated globally is below
                      this number.
            pressure  When the amount of memory  allocated  by  TCP  exceeds
                      this  number  of  pages, TCP moderates its memory con-
                      sumption.  This memory pressure state is  exited  once
                      the  number  of  pages  allocated  falls below the low
                      mark.
            high      The maximum number of pages, globally, that  TCP  will
                      allocate.   This  value  overrides  any  other  limits
                      imposed by the kernel.
     tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
            This parameter controls TCP Packetization-Layer Path MTU Discov-
            ery.  The following values may be assigned to the file:
            0  Disabled
            1  Disabled by default, enabled when an ICMP black hole detected
            2  Always enabled, use initial MSS of tcp_base_mss.
     tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
            By default, TCP saves various connection metrics  in  the  route
            cache  when  the  connection  closes, so that connections estab-
            lished in the near future can use these to  set  initial  condi-
            tions.   Usually, this increases overall performance, but it may
            sometimes cause performance degradation.  If tcp_no_metrics_save
            is enabled, TCP will not cache metrics on closing connections.
     tcp_orphan_retries (integer; default: 8; since Linux 2.4)
            The  maximum number of attempts made to probe the other end of a
            connection which has been closed by our end.
     tcp_reordering (integer; default: 3; since Linux 2.4)
            The maximum a packet can be reordered in  a  TCP  packet  stream
            without  TCP assuming packet loss and going into slow start.  It
            is not advisable to  change  this  number.   This  is  a  packet
            reordering  detection  metric  designed  to minimize unnecessary
            back off and retransmits provoked by reordering of packets on  a
            connection.
     tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
            Try to send full-sized packets during retransmit.
     tcp_retries1 (integer; default: 3; since Linux 2.2)
            The  number  of times TCP will attempt to retransmit a packet on
            an established connection normally, without the extra effort  of
            getting the network layers involved.  Once we exceed this number
            of retransmits, we first have the network layer update the route
            if  possible before each new retransmit.  The default is the RFC
            specified minimum of 3.
     tcp_retries2 (integer; default: 15; since Linux 2.2)
            The maximum number of times a TCP  packet  is  retransmitted  in
            established  state  before  giving up.  The default value is 15,
            which corresponds to a duration of approximately between  13  to
            30  minutes,  depending  on  the  retransmission  timeout.   The
            RFC 1122 specified minimum limit of  100  seconds  is  typically
            deemed too short.
     tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
            Enable TCP behavior conformant with RFC 1337.  When disabled, if
            a RST is received in TIME_WAIT state, we close the socket  imme-
            diately without waiting for the end of the TIME_WAIT period.
     tcp_rmem (since Linux 2.4)
            This  is  a  vector  of  3 integers: [min, default, max].  These
            parameters are used by TCP to  regulate  receive  buffer  sizes.
            TCP  dynamically adjusts the size of the receive buffer from the
            defaults listed below, in the range of these  values,  depending
            on memory available in the system.
            min       minimum  size  of  the receive buffer used by each TCP
                      socket.  The default value is the  system  page  size.
                      (On  Linux  2.4, the default value is 4 kB, lowered to
                      PAGE_SIZE bytes in low-memory systems.)  This value is
                      used  to  ensure that in memory pressure mode, alloca-
                      tions below this size will still succeed.  This is not
                      used  to bound the size of the receive buffer declared
                      using SO_RCVBUF on a socket.
            default   the default size of  the  receive  buffer  for  a  TCP
                      socket.   This  value  overwrites  the initial default
                      buffer    size     from     the     generic     global
                      net.core.rmem_default  defined for all protocols.  The
                      default value is 87380 bytes.   (On  Linux  2.4,  this
                      will  be  lowered to 43689 in low-memory systems.)  If
                      larger receive buffer sizes are  desired,  this  value
                      should  be  increased  (to  affect  all  sockets).  To
                      employ  large  TCP  windows,   the   net.ipv4.tcp_win-
                      dow_scaling must be enabled (default).
            max       the  maximum  size  of the receive buffer used by each
                      TCP socket.  This value does not override  the  global
                      net.core.rmem_max.  This is not used to limit the size
                      of the receive buffer declared using  SO_RCVBUF  on  a
                      socket.   The  default  value  is calculated using the
                      formula
                          max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
                      (On Linux 2.4, the default is 87380*2  bytes,  lowered
                      to 87380 in low-memory systems).
     tcp_sack (Boolean; default: enabled; since Linux 2.2)
            Enable RFC 2018 TCP Selective Acknowledgements.
     tcp_slow_start_after_idle   (Boolean;  default:  enabled;  since  Linux
     2.6.18)
            If  enabled,  provide RFC 2861 behavior and time out the conges-
            tion window after an idle period.  An idle period is defined  as
            the current RTO (retransmission timeout).  If disabled, the con-
            gestion window will not be timed out after an idle period.
     tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
            If this option is enabled, then use the RFC 1122  interpretation
            of  the TCP urgent-pointer field.  According to this interpreta-
            tion, the urgent pointer points to the last byte of urgent data.
            If  this  option is disabled, then use the BSD-compatible inter-
            pretation of the urgent pointer: the urgent  pointer  points  to
            the  first byte after the urgent data.  Enabling this option may
            lead to interoperability problems.
     tcp_syn_retries (integer; default: 5; since Linux 2.2)
            The maximum number of times initial SYNs for an active TCP  con-
            nection attempt will be retransmitted.  This value should not be
            higher than 255.  The default value is 5, which  corresponds  to
            approximately 180 seconds.
     tcp_synack_retries (integer; default: 5; since Linux 2.2)
            The  maximum number of times a SYN/ACK segment for a passive TCP
            connection will be retransmitted.  This  number  should  not  be
            higher than 255.
     tcp_syncookies (Boolean; since Linux 2.2)
            Enable  TCP  syncookies.   The kernel must be compiled with CON-
            FIG_SYN_COOKIES.  Send out syncookies when the syn backlog queue
            of  a socket overflows.  The syncookies feature attempts to pro-
            tect a socket from a SYN flood attack.  This should be used as a
            last  resort,  if at all.  This is a violation of the TCP proto-
            col, and conflicts with other areas of TCP such  as  TCP  exten-
            sions.  It can cause problems for clients and relays.  It is not
            recommended as a tuning mechanism for heavily loaded servers  to
            help  with  overloaded  or misconfigured conditions.  For recom-
            mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
            and tcp_abort_on_overflow.
     tcp_timestamps (integer; default: 1; since Linux 2.2)
            Set to one of the following values to enable or disable RFC 1323
            TCP timestamps:
            0  Disable timestamps.
            1  Enable timestamps as defined in RFC1323 and use random offset
               for  each connection rather than only using the current time.
            2  As for the value 1,  but  without  random  offsets.   Setting
               tcp_timestamps  to this value is meaningful since Linux 4.10.
     tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
            This parameter controls what percentage of the congestion window
            can  be  consumed  by  a  single  TCP Segmentation Offload (TSO)
            frame.  The setting of this  parameter  is  a  tradeoff  between
            burstiness and building larger TSO frames.
     tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
            Enable  fast  recycling  of  TIME_WAIT  sockets.   Enabling this
            option is not recommended as the remote IP may not use monotoni-
            cally  increasing  timestamps  (devices behind NAT, devices with
            per-connection timestamp offsets).  See RFC 1323 (PAWS) and  RFC
            6191.
     tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
            Allow  to reuse TIME_WAIT sockets for new connections when it is
            safe from protocol viewpoint.  It should not be changed  without
            advice/request of technical experts.
     tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
            Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
            sender-side-only change to TCP that  anticipates  the  onset  of
            congestion  by  estimating the bandwidth.  TCP Vegas adjusts the
            sending rate by modifying  the  congestion  window.   TCP  Vegas
            should  provide less packet loss, but it is not as aggressive as
            TCP Reno.
     tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
            Enable TCP Westwood+ congestion control  algorithm.   TCP  West-
            wood+  is a sender-side-only modification of the TCP Reno proto-
            col stack that optimizes the performance of TCP congestion  con-
            trol.   It  is  based  on end-to-end bandwidth estimation to set
            congestion window and slow start threshold  after  a  congestion
            episode.  Using this estimation, TCP Westwood+ adaptively sets a
            slow start threshold and a congestion window  which  takes  into
            account  the  bandwidth  used  at the time congestion is experi-
            enced.  TCP  Westwood+  significantly  increases  fairness  with
            respect  to TCP Reno in wired networks and throughput over wire-
            less links.
     tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
            Enable RFC 1323 TCP window scaling.  This feature allows the use
            of  a  large  window  (>  64 kB) on a TCP connection, should the
            other end support it.  Normally, the 16 bit window length  field
            in the TCP header limits the window size to less than 64 kB.  If
            larger windows are desired, applications can increase  the  size
            of  their  socket  buffers and the window scaling option will be
            employed.  If tcp_window_scaling is disabled, TCP will not nego-
            tiate  the  use of window scaling with the other end during con-
            nection setup.
     tcp_wmem (since Linux 2.4)
            This is a vector of 3  integers:  [min,  default,  max].   These
            parameters  are  used by TCP to regulate send buffer sizes.  TCP
            dynamically adjusts the size of the send buffer from the default
            values  listed below, in the range of these values, depending on
            memory available.
            min       Minimum size of the  send  buffer  used  by  each  TCP
                      socket.   The  default  value is the system page size.
                      (On Linux 2.4, the default value is 4 kB.)  This value
                      is  used to ensure that in memory pressure mode, allo-
                      cations below this size will still succeed.   This  is
                      not used to bound the size of the send buffer declared
                      using SO_SNDBUF on a socket.
            default   The default size of the send buffer for a TCP  socket.
                      This  value overwrites the initial default buffer size
                      from           the           generic            global
                      /proc/sys/net/core/wmem_default defined for all proto-
                      cols.  The default value is  16 kB.   If  larger  send
                      buffer   sizes  are  desired,  this  value  should  be
                      increased (to affect all sockets).   To  employ  large
                      TCP windows, the /proc/sys/net/ipv4/tcp_window_scaling
                      must be set to a nonzero value (default).
            max       The maximum size of the send buffer used by  each  TCP
                      socket.   This  value  does  not override the value in
                      /proc/sys/net/core/wmem_max.   This  is  not  used  to
                      limit  the  size  of  the  send  buffer declared using
                      SO_SNDBUF on a socket.  The default  value  is  calcu-
                      lated using the formula
                          max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
                      (On  Linux  2.4,  the default value is 128 kB, lowered
                      64 kB depending on low-memory systems.)
     tcp_workaround_signed_windows (Boolean; default: disabled; since  Linux
     2.6.26)
            If enabled, assume that no receipt of  a  window-scaling  option
            means  that  the remote TCP is broken and treats the window as a
            signed quantity.  If disabled, assume that the remote TCP is not
            broken  even  if  we do not receive a window scaling option from
            it.
 Socket options
     To set or get a TCP socket option, call getsockopt(2) to read  or  set-
     sockopt(2)  to  write  the option with the option level argument set to
     IPPROTO_TCP.  Unless otherwise noted, optval is a pointer  to  an  int.
     In  addition,  most IPPROTO_IP socket options are valid on TCP sockets.
     For more information see ip(7).
     TCP_CONGESTION (since Linux 2.6.13)
            The argument for this option is a string.   This  option  allows
            the  caller  to  set  the TCP congestion control algorithm to be
            used,  on  a  per-socket  basis.   Unprivileged  processes   are
            restricted to choosing one of the algorithms in tcp_allowed_con-
            gestion_control   (described   above).    Privileged   processes
            (CAP_NET_ADMIN) can choose from any of the available congestion-
            control algorithms (see the description of tcp_available_conges-
            tion_control above).
     TCP_CORK (since Linux 2.2)
            If  set,  don't  send  out  partial  frames.  All queued partial
            frames are sent when the option is cleared again.  This is  use-
            ful  for  prepending  headers before calling sendfile(2), or for
            throughput optimization.  As currently implemented, there  is  a
            200  millisecond  ceiling on the time for which output is corked
            by TCP_CORK.  If this ceiling is reached, then  queued  data  is
            automatically  transmitted.   This  option  can be combined with
            TCP_NODELAY only since Linux 2.5.71.  This option should not  be
            used in code intended to be portable.
     TCP_DEFER_ACCEPT (since Linux 2.4)
            Allow  a  listener  to be awakened only when data arrives on the
            socket.  Takes an integer value (seconds), this  can  bound  the
            maximum number of attempts TCP will make to complete the connec-
            tion.  This option should not be used in  code  intended  to  be
            portable.
     TCP_INFO (since Linux 2.4)
            Used  to  collect  information  about  this  socket.  The kernel
            returns   a   struct   tcp_info   as   defined   in   the   file
            /usr/include/linux/tcp.h.   This  option  should  not be used in
            code intended to be portable.
     TCP_KEEPCNT (since Linux 2.4)
            The maximum number of keepalive probes TCP  should  send  before
            dropping the connection.  This option should not be used in code
            intended to be portable.
     TCP_KEEPIDLE (since Linux 2.4)
            The time (in seconds) the connection needs to remain idle before
            TCP  starts  sending  keepalive  probes,  if  the  socket option
            SO_KEEPALIVE has been set on this socket.   This  option  should
            not be used in code intended to be portable.
     TCP_KEEPINTVL (since Linux 2.4)
            The time (in seconds) between individual keepalive probes.  This
            option should not be used in code intended to be portable.
     TCP_LINGER2 (since Linux 2.4)
            The lifetime of orphaned FIN_WAIT2 state sockets.   This  option
            can  be  used  to  override  the system-wide setting in the file
            /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This is not
            to  be confused with the socket(7) level option SO_LINGER.  This
            option should not be used in code intended to be portable.
     TCP_MAXSEG
            The maximum segment size for outgoing TCP packets.  In Linux 2.2
            and  earlier,  and  in Linux 2.6.28 and later, if this option is
            set before connection establishment, it  also  changes  the  MSS
            value  announced to the other end in the initial packet.  Values
            greater than the (eventual) interface MTU have no  effect.   TCP
            will  also  impose its minimum and maximum bounds over the value
            provided.
     TCP_NODELAY
            If set, disable the Nagle algorithm.  This means  that  segments
            are  always  sent  as  soon as possible, even if there is only a
            small amount of data.  When not  set,  data  is  buffered  until
            there  is  a sufficient amount to send out, thereby avoiding the
            frequent sending of small packets, which results  in  poor  uti-
            lization of the network.  This option is overridden by TCP_CORK;
            however, setting this option forces an explicit flush of pending
            output, even if TCP_CORK is currently set.
     TCP_QUICKACK (since Linux 2.4.4)
            Enable quickack mode if set or disable quickack mode if cleared.
            In quickack mode, acks are sent immediately, rather than delayed
            if  needed  in accordance to normal TCP operation.  This flag is
            not permanent, it only enables a  switch  to  or  from  quickack
            mode.   Subsequent operation of the TCP protocol will once again
            enter/leave quickack mode depending on  internal  protocol  pro-
            cessing  and  factors such as delayed ack timeouts occurring and
            data transfer.  This option should not be used in code  intended
            to be portable.
     TCP_SYNCNT (since Linux 2.4)
            Set  the  number  of SYN retransmits that TCP should send before
            aborting the attempt to connect.  It cannot  exceed  255.   This
            option should not be used in code intended to be portable.
     TCP_USER_TIMEOUT (since Linux 2.6.37)
            This  option  takes  an  unsigned  int as an argument.  When the
            value is greater than 0, it specifies the maximum amount of time
            in  milliseconds that transmitted data may remain unacknowledged
            before TCP will forcibly close the corresponding connection  and
            return  ETIMEDOUT  to  the  application.  If the option value is
            specified as 0, TCP will to use the system default.
            Increasing user timeouts allows  a  TCP  connection  to  survive
            extended  periods  without  end-to-end connectivity.  Decreasing
            user timeouts allows applications to "fail fast", if so desired.
            Otherwise,  failure  may  take up to 20 minutes with the current
            system defaults in a normal WAN environment.
            This option can be set during any state of a TCP connection, but
            is effective only during the synchronized states of a connection
            (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,  CLOSING,  and
            LAST-ACK).    Moreover,   when   used  with  the  TCP  keepalive
            (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will override  keepalive
            to  determine  when to close a connection due to keepalive fail-
            ure.
            The option has no effect on when TCP retransmits a  packet,  nor
            when a keepalive probe is sent.
            This  option,  like many others, will be inherited by the socket
            returned by accept(2), if it was set on the listening socket.
            Further details on the user timeout  feature  can  be  found  in
            RFC 793 and RFC 5482 ("TCP User Timeout Option").
     TCP_WINDOW_CLAMP (since Linux 2.4)
            Bound the size of the advertised window to this value.  The ker-
            nel imposes a minimum size of  SOCK_MIN_RCVBUF/2.   This  option
            should not be used in code intended to be portable.
 Sockets API
     TCP  provides  limited  support for out-of-band data, in the form of (a
     single byte of) urgent data.  In Linux this  means  if  the  other  end
     sends  newer out-of-band data the older urgent data is inserted as nor-
     mal data into the stream (even when SO_OOBINLINE  is  not  set).   This
     differs from BSD-based stacks.
     Linux  uses  the  BSD  compatible  interpretation of the urgent pointer
     field by default.  This violates RFC 1122, but is required for interop-
     erability    with    other    stacks.     It   can   be   changed   via
     /proc/sys/net/ipv4/tcp_stdurg.
     It is possible to peek at out-of-band data using the  recv(2)  MSG_PEEK
     flag.
     Since  version  2.4,  Linux  supports the use of MSG_TRUNC in the flags
     argument of recv(2) (and recvmsg(2)).  This flag  causes  the  received
     bytes of data to be discarded, rather than passed back in a caller-sup-
     plied buffer.  Since Linux 2.4.4, MSG_TRUNC also has this  effect  when
     used in conjunction with MSG_OOB to receive out-of-band data.
 Ioctls
     The  following ioctl(2) calls return information in value.  The correct
     syntax is:
            int value;
            error = ioctl(tcp_socket, ioctl_type, &value);
     ioctl_type is one of the following:
     SIOCINQ
            Returns the amount of queued unread data in the receive  buffer.
            The socket must not be in LISTEN state, otherwise an error (EIN-
            VAL) is returned.   SIOCINQ  is  defined  in  <linux/sockios.h>.
            Alternatively,  you  can use the synonymous FIONREAD, defined in
            <sys/ioctl.h>.
     SIOCATMARK
            Returns true (i.e., value is nonzero) if the inbound data stream
            is at the urgent mark.
            If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
            true, then the next read from the socket will return the  urgent
            data.  If the SO_OOBINLINE socket option is not set, and SIOCAT-
            MARK returns true, then the  next  read  from  the  socket  will
            return the bytes following the urgent data (to actually read the
            urgent data requires the recv(MSG_OOB) flag).
            Note that a read never reads across  the  urgent  mark.   If  an
            application  is  informed  of  the  presence  of urgent data via
            select(2) (using the exceptfds argument) or through delivery  of
            a SIGURG signal, then it can advance up to the mark using a loop
            which repeatedly tests SIOCATMARK and performs a read  (request-
            ing any number of bytes) as long as SIOCATMARK returns false.
     SIOCOUTQ
            Returns the amount of unsent data in the socket send queue.  The
            socket must not be in LISTEN state, otherwise an error  (EINVAL)
            is  returned.  SIOCOUTQ is defined in <linux/sockios.h>.  Alter-
            natively, you  can  use  the  synonymous  TIOCOUTQ,  defined  in
            <sys/ioctl.h>.
 Error handling
     When  a  network  error  occurs, TCP tries to resend the packet.  If it
     doesn't succeed after some time, either ETIMEDOUT or the last  received
     error on this connection is reported.
     Some  applications  require  a quicker error notification.  This can be
     enabled with the IPPROTO_IP level IP_RECVERR socket option.  When  this
     option  is  enabled,  all incoming errors are immediately passed to the
     user program.  Use this option with care -- it makes TCP less  tolerant
     to routing changes and other normal network conditions.

ERRORS

     EAFNOTSUPPORT
            Passed socket address type in sin_family was not AF_INET.
     EPIPE  The  other  end closed the socket unexpectedly or a read is exe-
            cuted on a shut down socket.
     ETIMEDOUT
            The other end didn't acknowledge retransmitted data  after  some
            time.
     Any  errors  defined  for ip(7) or the generic socket layer may also be
     returned for TCP.

VERSIONS

     Support for Explicit Congestion  Notification,  zero-copy  sendfile(2),
     reordering  support and some SACK extensions (DSACK) were introduced in
     2.4.  Support for forward acknowledgement (FACK), TIME_WAIT  recycling,
     and per-connection keepalive socket options were introduced in 2.3.

BUGS

     Not all errors are documented.
     IPv6 is not described.

SEE ALSO

     accept(2),  bind(2),  connect(2), getsockopt(2), listen(2), recvmsg(2),
     sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)
     RFC 793 for the TCP specification.
     RFC 1122 for the TCP requirements and a description of the Nagle  algo-
     rithm.
     RFC 1323 for TCP timestamp and window scaling options.
     RFC 1337 for a description of TIME_WAIT assassination hazards.
     RFC 3168 for a description of Explicit Congestion Notification.
     RFC 2581 for TCP congestion control algorithms.
     RFC 2018 and RFC 2883 for SACK and extensions to SACK.

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 TCP(7)

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